1. Field of the Invention
The present invention relates to methods of preparing polymer modified pigments, as well as polymer modified pigments and inkjet ink compositions comprising the same.
2. Description of the Related Art
The surface of pigments contains a variety of different functional groups, and the types of groups present depend on the specific class of pigment. Several methods have been developed for grafting materials and, in particular, polymers, to the surface of these pigments. For example, it has been shown that polymers can be attached to carbon blacks containing surface groups such as phenols and carboxyl groups. However, methods that rely on the inherent functionality of a pigment's surface cannot be applied generally because not all pigments have the same specific functional groups.
Other methods for the preparation of polymer modified pigment products have also been developed. For example, U.S. Pat. Nos. 7,056,962, 6,478,863, 6,432,194, 6,336,965, and as well as U.S. Patent Application Publication No. 2006-0189717 describe methods for attaching polymers to pigments through the use of a diazonium salt. Also, U.S. Pat. Nos. 7,173,078, 6,916,367, 6,911,073, 6,723,783, 6,699,319, 6,472,471, and 6,110,994 disclose methods of preparing a polymer modified pigment by reacting polymer and a pigment having an attached reactive group. Furthermore, modified pigments having attached polymeric groups have also been disclosed in U.S. Patent Application Publication No. 2008-0177003 Al, which utilizes a polymer in the form of a melt.
In general, each of these related to a method of preparing pigments having attached at least one polymeric group sometimes referred to as a “grafting onto” process, which generally involves the reaction of polymeric materials having reactive functional groups onto the surface of a particle, such as a pigment. For this type of process, polymer that has reacted with the surface may cause steric hindrance, thereby preventing additional polymeric material from reaching the surface of the pigment and limiting the amount of polymer attached to the pigment surface. In general, such approaches have a lower level of efficiency of polymer attachment (expressed as a percentage of attached polymer versus the amount of polymer added). Thus, to obtain a pigment having a high level of attached polymer, a large excess of polymer is needed.
Alternative methods for attaching polymeric groups to a pigment include so-called “grafting through” or “grafting from” processes. A “grafting through” method generally involves the polymerization of monomers in the presence of a modified pigment having attached at least one polymerizable group. However, similar to the “grafting to” method, the presence of attached polymer may limit further attachment since the attached polymer may sterically hinder the growing polymer chains from reaching the polymerizable group on the pigment surface, thus lowering the efficiency of the attachment.
By comparison, a “grafting from” process typically comprises forming initiation sites on the surface of the pigment and polymerizing monomers directly from the initiation site. Examples of such polymerization processes include atom transfer radical polymerization (ATRP), stable free radical (SFR) polymerization, and reversible addition-fragmentation chain transfer polymerization (RAFT), as well as ionic polymerizations such as group transfer polymerization (GTP). While such methods typically afford a higher grafting density due to the much higher diffusion rate of small molecules (i.e., monomers) as compared to polymers in the “grafting onto” or “grafting through” processes, these methods also generally require the preparation of a specific initiator-modified pigments along with the use of special reaction conditions to grow the polymer, adding additional cost and complexity to the process.
Therefore, while these methods provide modified pigments having attached polymeric groups, there remains a need for improved methods for attaching polymers to pigments, with improved attachment efficiency, as well as polymer modified pigments having higher levels of attached polymer.